Combustion furnaces are widely used. In said furnaces, heat is generated by the combustion of one or more fuels with an appropriate oxidizer.
Such combustion furnaces are generally operated with air as oxidizer. It is also known to use oxygen-enriched air or oxygen as oxidizer in combustion furnaces.
U.S. Pat. No. 5,587,283 describes a combustion process using oxygen as oxidizer. According to said process, a single combustible gas, such as natural gas, and oxygen are injected via a burner, 15 to 35% of the total oxygen being injected as a coherent jet in the liquid phase. The process is said to be suitable for use in metal melting furnaces, enamel furnaces and glass furnaces. By injecting the liquefied oxygen as a coherent jet at high velocity, said oxygen passes through the high temperature region for the downstream combustion of the remainder of the gaseous fuel. An elongation of the flame is obtained which is adjustable as a function of the proportion of liquefied gas injected.
A further combustion process in which liquid oxygen is used is described in EP-A-0866295. EP-A-0866295 discloses more specifically a method in which preheated mineral feed material is charged into a rotary kiln and thermally processed by a flame produced by combustion of a single unidentified fuel with an oxygen containing gas, and whereby at least one jet of liquid oxygen is injected into the kiln, in particular beneath the flame, so that the oxygen impacts the mineral feed material in liquid form. The liquid oxygen then accompanies the charge material as it tumbles down inside the rotary kiln towards the flame.
Combustion furnaces are generally preferably operated with highly flammable fuels such as natural gas and fuel oil, but some processes make use of or require the combustion of low grade fuels or combustibles of low flammability, including waste products.
For example, for the production of cement in a rotary tubular kiln, it is known to generate a flame with a conventional high calorific value fuel. The temperature of said flame can be increased by supplying additional oxidizer, for example oxygen-enriched air, on top of the hot coolant air coming from the clinker cooler.
In order to keep the costs for fuel low, it has become an established practice to add an inexpensive secondary fuel to the primary fuel. The calorific value of the secondary fuel is lower than the calorific value of the primary fuel and the secondary fuel burns in the flame of the primary fuel. In particular, use is sometimes made of plastics waste as the secondary fuel. This plastics waste is injected into the rotary tubular kiln as shredded particles such that it burns as completely as possible in the flame of the primary fuel. The secondary fuel therefore comprises constituents which do not all have the same geometry.
It has been discovered that not all the constituents of the secondary fuel burn completely. Instead, the secondary fuel constituents which are not completely burnt generate soot particles, and impair the quality of the cement which is to be produced. In addition, incomplete combustion can lead to the formation of carbon monoxide, which where possible should not pass into the environment.
EP-B-1 065 461 discloses a calcination process adapted to enable the combustion of such a low-calorific-value secondary fuel. In said process, the mineral material to be calcined is heated by means of a flame comprising a primary combustion zone and a downstream secondary combustion zone. The primary combustion zone is created by the combustion of a primary fuel with a first oxidizer and is located near the points of injection of the first oxidizer and of the primary fuel. The flame further comprises a secondary combustion zone located downstream of the primary zone and created by the combustion of a secondary fuel with a secondary oxidizer, the secondary fuel being preheated by flowing through the primary zone of the flame before entering the secondary combustion zone. The secondary fuel typically has a low calorific value of 15×106 J/kg or less. The first oxidizer has an oxygen content of more than 21% and up to substantially 100%. The primary oxidizer may have a temperature of about 100° C. The secondary oxidizer is preferably air and in particular air coming from the clinker cooler and having a temperature of between 500° C. and 1000° C. Both oxidizers are therefore used in gaseous form.